ARM: dts: imx6dl-plybas: Use the standard 'uart-has-rtscts'

The usage of the 'fsl,uart-has-rtscts' property is deprecated.

Use the standard 'uart-has-rtscts' instead.

Cc: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>